The definition of a tile being resolved is that local changes have been flushed out to the surface, hence there is no need to reload the tile before it's written to. For an invalid tile, the tile has to be reloaded from the surface before rendering.

Stage (2) was marking hot tiles for attachements 0 & 1 as RESOLVED, which means that the hot tiles can be written out to memory with no need to read them back in (they are "clean"). They need to be marked as resolved here, because a surface may be destroyed after a detach, and we don't want to have un-resolved tiles that may force a readback from a NULL (destroyed) surface. (Part of a destroy is detach all attachments first)

Stage (3), during the no att -> att transition, we need to realize that the "new" surface tiles need to be fetched fresh from the new surface, instead of using the resolved tiles, that belong to a stale attachment.

This is done by marking the hot tiles as invalid in stage (3), when we realize that a new attachment is being made, so that they are re-fetched during rendering in stage (4).